Harnessing the unlimited energetic potential of sunlight is one of the cornerstones of current efforts to arrive at future solutions to global energy problems. These problems are (primarily) twofold, namely the pollution and evolution of greenhouse gases associated with burning fossil fuels (85% of the 13 TW of yearly global energy production is based on combustion), and the fact that such fuels are limited and will become unavailable in the not too distant future.
Photovoltaic power generation and solar thermal power generation are currently the two most widely exploited methods for harnessing solar power. Photovoltaic power generation transforms solar energy directly to electricity without generating heat. In contrast, solar thermal power generation systems rely on concentrating sunlight to generate heat, which can be converted to power using gas or steam turbines via Brayton cycle or Rankine cycle mechanisms, respectively. Concentration of solar energy is accomplished by parabolic troughs or by mirrors that direct sunlight to a collector tower. The concentrated energy can be used, for example, to heat a boiler atop the collector tower and generate steam that is piped into a turbine, where electricity can be produced. Storage of the sun's heat in these systems is accomplished by heating and storing molten salts, such as sodium and potassium nitrates.
Unfortunately, traditional solar thermal power generation has a number of limitations which include the necessity to track and adjust the mirrors throughout the day in order to keep the focused light on the collector tower, and the necessity to use or convert the obtained thermal power on-site. This is because current solar thermal power storage media (e.g., molten salts) are not easily amenable to transportation over long distances due to heat losses, and, generally, have a rather limited duration of heat storage. In addition, high-temperature heat storage which is often used during power generation to increase efficiency, leads to even faster heat loss.
A conceptually different approach would be to store sun light energy in a molecular system. In this system, a lower energy compound is photoconverted to a higher energy isomer, which, in turn, is converted back to the lower energy isomer with release of thermal energy. The best known purely organic examples of such processes are the norbornadiene (1)—quadricyclane (2) and the stilbene (3)—dihydrophenanthrene (4) cycles (reactions 1 and 2 respectively). While much has been learned from such and other prototypes, they all have drawbacks to varying degrees, among them photoinstability, low quantum yields, structural restrictions of the absorbing light frequency range, and limited capacity for energy storage. These limitations have precluded industrial applicability of such systems.

In 1983, a first organometallic compound that was capable of photoconversion to its higher-energy isomer and reverse exothermic isomerization was reported by Vollhardt et al. (J. Am. Chem. Soc. 1983, 105, 1676-1677). In this system, as shown in equation 3, a photochemically active fulvalenyl diruthenium complex 5 is converted to a higher-energy isomer 6 upon exposure to sunlight. The higher-energy isomer 6 can be converted back to the low-energy isomer 5 upon heating of 6. This reverse reaction is accompanied by a significant release of thermal energy. It was determined that conversion of 6 to 5 releases about 30 kcal/mol of energy.

However, to date, organometallic assemblies shown in equation 3 have not been commercialized for solar energy storage.